The purpose of this project is to study changes in the ultrastructure of the hair cells of the organ of Corti. The present project is designed to study the ultrastructural and electrophysiological changes of hair cells after alteration of the ionic concentration of endolymph in the chinchilla cochlea. Perfusion of the scala media with artificial perilymph resulted in a progressive decline of the cochlear potentials but there was no clear increase in the number of vesicles associated with the presynaptic membrane.